1. Field of the Invention
The present invention relates generally to patient monitoring systems and more specifically to patient fluid monitoring systems.
2. Discussion of the Prior Art
Human patients undergoing surgery lose fluids which require replacement at rates depending upon the location and type of surgery. Peripheral procedures require the least fluids while intra-abdominal procedures require the most. Some procedures even introduce more fluid into patients than is removed. Procedures such as transurethral bladder and prostate resections, and hysteroscopic vaginal hysterectomies using large volumes of irrigating solutions may result in absorption of significant amounts of the irrigating solution accompanied by increased intravascular volume and the dangers of congestive heart failure and hyponatremia (see J. C. Ayus & A. I. Arieff, Glycine-induced Hypoosmolar Hyponatremia, 157 Arch. Intern. Med. 223 (1997), which is hereby incorporated by reference).
Much of the rationale for administering large quantities of postoperative fluids, despite the attendant dangers of pulmonary edema, include the effects of the "third space," regions in the body occupied by fluids which are not in equilibrium with the bloodstream. Examples of the third space include burns, bruises, traumatized operative bed (intra-abdominal or intra-thoracic), traumatically injured tissues, and infarcted tissues. Fluids in the third space are literally out of circulation and therefore hemodynamically inactive. Fluid sequestration in the third space is a unique kind of fluid loss in postoperative patients (see M. H. Rosenthal & A. I. Arieff, Fluid and Electrolyte Therapy in Critically Ill Patients and Those Who Are Pre-, Post-, or Intraoperative, in Fluid, Electrolyte and Acid-Base Disorders 597 (A. I. Arieff & R. A. DeFronzo eds., 1995), which is hereby incorporated by reference).
However, there is no simple bedside method for accurately measuring these fluid losses, and in actual practice clinical approximation determines replacement. The sequestered extracellular fluid (third space losses) postoperatively during an uncomplicated procedure varies between negligible and 3 liters. Quantification of functional extracellular fluid using the available means of measuring is extremely difficult, and consequently no accurate a priori formula for intraoperative fluid administration has been derived.
Postoperative fluid balance depends on underlying pathology factors including anesthesia, intraoperative fluid therapy and intra- and post-operative complications. Humoral mediators (such as the renin-angiotensin system, catecholamines, aldosterone, and AVP), which can influence hemodynamics and are released during surgery as described below, may persist into the postoperative period and require continuous administration of large volumes of fluids. While the intravascular volume must be maintained to avoid postoperative renal insufficiency, too much postoperative fluid can result in heart and lung failure with pulmonary edema. The potential postoperative complication of pulmonary edema and respiratory failure is a major hazard which discourages administering fluids in sufficient quantities to maintain preload.
The quantity of fluid necessary to induce pulmonary edema varies according to individual patient factors such as age, body weight, tissue turgor, cardiac function, pulmonary function, renal function, plasma vasopressin levels, and plasma proteins. The literature includes some information concerning minimal quantities of fluid which could induce pulmonary edema in otherwise generally healthy postoperative patients, but this information does not imply that any given quantity of fluid will necessarily induce pulmonary edema. Little information is available concerning the maximum postoperative volume of fluid which can be safely administered. In particular, it is not clear what volume of fluid might result in pulmonary edema in a postoperative patient who does not have serious cardiovascular, hepatic or renal disorders.
FIG. 1 is a bar chart illustrating the incidence of pulmonary edema among a total of 161 patients who retained an average of 2.2 liters of fluid per day following surgery.
Postoperative hyponatremia is the most frequent postoperative electrolyte complication among adults in the United States and in the United Kingdom. Every post operative patient should be considered at risk to develop hyponatremia. (see C. L. Fraser & A. I. Arieff, Epidemiology, Pathophysiology, and Management of Hyponatremic Encephalopathy, 102 Am. J. Med. 67 (1997); R. Zerbe & G. Robertson, Osmotic and Nonosmotic Regulation of Thirst and Vasopressin Secretion, in Clinical Disorders of Fluid and Electrolyte Metabolism 81 (R. G. Narins ed., 1994); A. I Arieff, J. C. Ayus, & C. L. Fraser, Hyponatremia and Death or Permanent Brain Damage in Healthy Children, 304 Brit. Med. J. 1218 (1992); and J. C. Ayus, J. M. Wheeler, & A. I. Arieff, Postoperative Hyponatremic Encephalopathy in Menstruant Women, 117 Ann. Intern. Med. 891 (1992), which are hereby incorporated by reference).
FIG. 2 shows that, among the approximately 25 million annual inpatient surgeries per year in the United States, an incidence of about 1% of postoperative hyponatremia results in about 250,000 cases per year. It has recently been projected that of postoperative patients who develop hyponatremic encephalopathy about 25% (above 12,000 per year in the USA) eventually die or suffer permanent brain damage. (see J. C. Ayus & A. I. Arieff, Brain Damage and Postoperative Hyponatremia: Role of Gender, 46 Neurology 323 (1996), which is hereby incorporated by reference).
The incidence of hypernatremia among all hospitalized patients is about 1.5% (375,000 patients) and about 21% of this total are postoperative (about 80,000 patients). If only isotonic fluids (154 mmol/L NaCl) are administered to postoperative patients, hypernatremia may develop (see N. A. Snyder & A. I. Arieff, Neurological Manifestations of Hypernatremia, in Metabolic Brain Dysfunction in Systematic Disorders 87 (R. A. Griggs & A. I. Arieff, eds., 1992), which is hereby incorporated by reference). In postoperative patients, hypernatremia is caused by a relative lack of free water, and is associated with a mortality rate in excess of 40% (about 40,000 deaths) as shown in FIG. 3 (Id.; see also P. M. Palevsky, R. Bhagrath, & A. Greenberg, Hypernatremia in Hospitalized Patients, 124 Ann. Intern. Med. 197 (1996), which is hereby incorporated by reference).
Thus, as shown in FIG. 4, three major postoperative complications, hypematremia, hyponatremia, and pulmonary edema affect almost 650,000 postoperative patients, with an estimated mortality of 78,000 individuals, per year in the USA (see M. H. Rosenthal & A. I. Arieff, Fluid and Electrolyte Therapy in Critically Ill Patients and Those Who Are Pre-, Post-, or Intraoperative, in Fluid, Electrolyte and Acid-Base Disorders 597-632 (A. I. Arieff & R. A. DeFronzo eds., Churchill Livingstone, New York, 1995).
The art related to the field of systematic monitoring of the fluid and electrolyte balances in patients includes partial solutions to the above-described problems. There is no prior art comprehending a systematic approach which can warn physicians when a major problem (hyponatremia, hypernatremia, pulmonary edema) is imminent, and give meaningful suggestions to an attending physician. In many hospitals' operating rooms the fluid input and output volumes are roughly estimated by an attending physician aided only by his or her visual observations and experience.
Among prior inventions directed towards certain aspects of the fluid and electrolyte balance problem, Parrish (U.S. Pat. No. 4,448,207) and Blankenship, et al. (U.S. Pat. No. 4,658,834) both disclose apparatuses using sonic transducers for measuring of the volume of fluids outgoing from a patient. Corbitt, et al. (U.S. Pat. No. 4,449,538) discloses an apparatus which measures bulk fluid input and output volumes and advises an attending physician, but not on electrolyte balance. Cormier, et al. (U.S. Pat. No. 4,531,088) discloses in-line blood analysis through electrical resistance measuring, and Oppenheimer (U.S. Pat. No. 5,331,958) does the same through laser beams. Micklish (U.S. Pat. No. 5,285,682) addresses the problem of measuring the volume of fluid absorbed in sponges. Ludwigsen (U.S. Pat. No. 5,236,664) addresses the problem of losing blood in non-fluid forms by measuring levels of hemoglobin in blood-containing materials to estimate total blood loss.
The above inventions are useful in solving part of the problem of advising an attending physician about fluid and electrolyte balances in a patient, but the task of tying together the data in real time is left to an attending physician and members of the staff. Due to other demands on the critical attention of these people, attention to the underlying problem of maintaining a proper fluid and electrolyte balance is all too often lacking. None of the above inventions are designed to monitor changes in body electrolyte balance postoperatively. Two of these problems, hyponatremia and hypernatremia, are responsible for in excess of 50,000 deaths in the USA per year.